1,114 research outputs found
Optimism Experiment and Development of Space-qualified Seismometers in France
The OPTIMISM experiment will put two magnetometers and two seismometers on the Martian floor in 1995, within the framework of the Mars '94 mission. The seismometers are put within the two small surface stations. The seismometer sensitivity will be better than 10 exp -9 g at 1 Hz, 2 orders of magnitude higher than the Viking seismometer sensitivity. A priori waveform modeling for seismic signals on Mars shows that it will be sufficient to detect quakes with a seismic moment greater than 10 exp 15 Nm everywhere on Mars. Such events, according to the hypothesis of a thermoelastic cooling of the Martian lithosphere, are expected to occur at a rate close to one per week and may therefore be observed within the l-year lifetime of the experiment. Other aspects of the experiment are discussed
Ising Quantum Hall Ferromagnet in Magnetically Doped Quantum Wells
We report on the observation of the Ising quantum Hall ferromagnet with Curie
temperature as high as 2 K in a modulation-doped (Cd,Mn)Te
heterostructure. In this system field-induced crossing of Landau levels occurs
due to the giant spin-splitting effect. Magnetoresistance data, collected over
a wide range of temperatures, magnetic fields, tilt angles, and electron
densities, are discussed taking into account both Coulomb electron-electron
interactions and sd coupling to Mn spin fluctuations. The critical behavior
of the resistance ``spikes'' at corroborates theoretical
suggestions that the ferromagnet is destroyed by domain excitations.Comment: revised, 4 pages, 4 figure
Engineering of spin-lattice relaxation dynamics by digital growth of diluted magnetic semiconductor CdMnTe
The technological concept of "digital alloying" offered by molecular-beam
epitaxy is demonstrated to be a very effective tool for tailoring static and
dynamic magnetic properties of diluted magnetic semiconductors. Compared to
common "disordered alloys" with the same Mn concentration, the spin-lattice
relaxation dynamics of magnetic Mn ions has been accelerated by an order of
magnitude in (Cd,Mn)Te digital alloys, without any noticeable change in the
giant Zeeman spin splitting of excitonic states, i.e. without effect on the
static magnetization. The strong sensitivity of the magnetization dynamics to
clustering of the Mn ions opens a new degree of freedom for spin engineering.Comment: 9 pages, 3 figure
Optical manipulation of a single Mn spin in a CdTe-based quantum dot
A system of two coupled CdTe quantum dots, one of them containing a single Mn
ion, was studied in continuous wave and modulated photoluminescence,
photoluminescence excitation, and photon correlation experiments. Optical
writing of information in the spin state of the Mn ion has been demonstrated,
using orientation of the Mn spin by spin-polarized carriers transferred from
the neighbor quantum dot. Mn spin orientation time values from 20 ns to 100 ns
were measured, depending on the excitation power. Storage time of the
information in the Mn spin was found to be enhanced by application of a static
magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple
rate equation models were found to describe correctly static and dynamical
properties of the system.Comment: 4 pages, 3 figure
Optical Studies of Zero-Field Magnetization of CdMnTe Quantum Dots: Influence of Average Size and Composition of Quantum Dots
We show that through the resonant optical excitation of spin-polarized
excitons into CdMnTe magnetic quantum dots, we can induce a macroscopic
magnetization of the Mn impurities. We observe very broad (4 meV linewidth)
emission lines of single dots, which are consistent with the formation of
strongly confined exciton magnetic polarons. Therefore we attribute the
optically induced magnetization of the magnetic dots results to the formation
of spin-polarized exciton magnetic polarons. We find that the photo-induced
magnetization of magnetic polarons is weaker for larger dots which emit at
lower energies within the QD distribution. We also show that the photo-induced
magnetization is stronger for quantum dots with lower Mn concentration, which
we ascribe to weaker Mn-Mn interaction between the nearest neighbors within the
dots. Due to particular stability of the exciton magnetic polarons in QDs,
where the localization of the electrons and holes is comparable to the magnetic
exchange interaction, this optically induced spin alignment persists to
temperatures as high as 160 K.Comment: 26 pages, 7 figs - submitted for publicatio
Quantum Hall states under conditions of vanishing Zeeman energy
We report on magneto-transport measurements of a two-dimensional electron gas
confined in a CdMnTe quantum well structure under
conditions of vanishing Zeeman energy. The electron Zeeman energy has been
tuned via the exchange interaction in order to probe different quantum
Hall states associated with metallic and insulating phases. We have observed
that reducing Zeeman energy to zero does not necessary imply the disappearing
of quantum Hall states, i.e. a closing of the spin gap. The spin gap value
under vanishing Zeeman energy conditions is shown to be dependent on the
filling factor. Numerical simulations support a qualitative description of the
experimental data presented in terms of a crossing or an avoided-crossing of
spin split Landau levels with same orbital quantum number
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